In full-duplex hands-free (FDHF) audio communication systems where, by definition, both communication paths are open at all time, howling can be a serious issue. Howling, which is also known as squealing or singing, can emerge in closed electro-acoustic loops when the total loop gain exceeds 0 dB at certain frequencies. In a typical FDHF communication system, such as shown in FIG. 1, echo paths 10 and 12, directly between the loudspeaker 14 and microphone 16, and due to the reverberant characteristics of the room in which the telephones A and B are located, respectively, are present at both the near and far ends. These echo, or coupling, paths create feedback loops. In systems where a digital-analog hybrid 18, as shown in FIG. 2, is present, the communication path introduces some feedback as a result of impedance mismatch. For instance, with analog hands-free telephones, the analog interface at the connection point to the public-switch telephone creates such a loop 20 on all calls. With telephones providing only half-duplex hands-free functionality, howling is not generally an issue because of the strong gain attenuation that is applied to at least one of the receive or transmit directions at any given time.
Prior techniques addressing the problem of howling in electro-acoustic loops in general, and in hands-free communications systems in particular, can be divided into four approaches. These approaches can be used on full-band signals, sub-band signals, or in the frequency domain as in WO 99/026355 to Farhang-Boroujeny, entitled Acoustic Echo Cancellation Equipped With Howling Suppressor And Double-Talk Detector.
The first approach uses switched loss based on uni-directional or bi-directional speech activity. An example of this approach can be found in JP 2001-016342 to Masuda et al., entitled Voice Changeover Device. The principle is to apply some gain attenuation to one or both directions depending on voice activity detection states, such as single talk, double-talk or double-silence. Essentially, the transmit gain is reduced when a receive signal is detected, or vice versa, and both transmit and receive gains are attenuated by a lesser degree in detected double-talk and double-silence states. This essentially means that the device operates as a hybrid between full-duplex and half-duplex, a concept sometimes referred to as “partially duplex”.
In commercial phones, the proximity of the loudspeaker to the microphone, combined with the high gains that are needed to deliver sufficient volume to the users, result in considerable positive gain in the loop, typically in the range of 12-15 dB on each side. Therefore, the amount of switched loss that has to be applied to keep the loop stable is quite large, producing a significant half-duplex behavior of the telephone. In order to reduce this effect, the amount of loss can be decreased once the acoustic echo cancellation (AEC) filter has converged, relying on the cancellation provided by the AEC filter to keep the loop stable. This works well as long as the AEC filter does indeed provide significant echo cancellation. However, if the echo path changes, which happens for instance when the telephone user moves his hand between the loudspeaker and the microphone, or in the vicinity of the telephone, then the AEC filter does not any longer provide sufficient echo cancellation, and the loop can become unstable.
A second approach uses switched loss based on howling detection. Examples of this approach can be found in JP-11008896 to Tanaka et al., entitled Howling Prevention Device; and U.S. Pat. No. 6,590,974 to Remes, entitled Howling Controller. The general strategy in this approach is to design a howling detector, which can be a narrow-band signal detector or a more involved detector taking into account the dynamic behavior of the signal. Some amount of loss is then applied to one direction of the loop when the howling detector triggers. Other possible responses to the detection of a howling signal include forcing or speeding up the speed of adaptation of the AEC filter to help cancel feedback from the acoustic path.
The problem with this approach is that howling signals can present temporal and spectral that are very similar to speech signals. This is particularly true with high positive gains throughout a wide frequency range, because in such a case, the traditional assumption that howling is a narrow-band signal is not necessarily a valid assumption. It is, therefore, challenging to design a howling detector that combines low probability of miss and low probability of false trigger on speech. As a result, howling signals might not be detected fast enough or with a low enough probability of false detection on speech.
A third approach uses adaptive notch filters, as exemplified in JP 62-278898 to Honma et al., entitled Loudspeaker; J. Chambers, A. Constantinides, “Frequency tracking using constrained adaptive notch filters synthesised from allpass sections,” IEEE Proceedings, Vol. 137, Pt. F. No. 6, December 1990; and S. Kuo, J. Chen, “New adaptive IIR notch filter and its application to howling control in speakerphone systems,” Electronic Letters, Vol. 28, no 8, April 1992. The principle is to use an adaptive filter to detect and cancel periodic components of the signal in either direction of the loop. This can be interpreted as implicit periodic signal detection, since the convergence and coefficients of the adaptive filter do give information about the presence of periodic, or correlated, components in the signal. Many different techniques have been proposed using FIR, direct IIR, and cascaded adaptive filters. These techniques have been used to remove periodic components from a signal, or to remove line noise from periodic signals, a technique often referred to as adaptive line enhancement.
Although some level of improvement can be achieved using this approach, it is also limited when high positive gains are present in the loop throughout a wide frequency range because, as described above, howling signals may not be narrow-band, but can then present speech-like spectral characteristics. It is therefore very difficult, if not impossible, to design an adaptive notch filter that can attenuate the howling signals in such conditions without also attenuating parts of valid speech signals, and, thereby, introducing a penalty on the speech quality.
A fourth approach to controlling howling is to apply a frequency shift to the signal. Examples of this approach can be found in JP 62-278898 to Honma et al., entitled Loudspeaker; and M. Schroeder, “Improvement of acoustic-feedback stability by frequency shifting,” J. Acoust. Soc. Am., 36(9), 1718-1724, 1964. The principle is to shift the spectrum of the signal in one or both directions of the loop by a few Hertz (e.g. 5 to 10 Hz to avoid significant deterioration in the speech quality). Because howling is due to frequencies being “caught” and amplified in the loop, this frequency shift has the effect of passing the recurring signal through the local peaks and valleys of the frequency-domain transfer function of the loop, thereby averaging the local extrema. Although this technique can help if only localized maxima of the feedback transfer function are above the critical point of 0 dB, it does not bring a significant improvement when high positive gains are present in the loop throughout a wide frequency range. Therefore, the frequency shift approach is not a satisfactory solution to controlling howling.
It is, therefore, desirable to provide a system and method for controlling howling in FDHF communication systems that does not compromise the full-duplex capabilities of system, that is not dependent on howling detection, and that is effective in situations where high positive gains are present in the electro-acoustic loop over a wide frequency range.